1. Field of the Invention
The subject matter presented herein relates generally to the field of communications networks, and more particularly, to wireless mesh networks.
2. Description of Related Art
Mesh networks have proven very successful, and many examples of large, geographically distributed networks exist. The architecture of these networks typically supports a process control model where there are just a few nodes, known as access points (APs) or gateways, which provide access into and egress from the mesh network. A multiplicity of endpoint nodes in the mesh network can be accessed from these access point entry nodes. Requests and commands may be sent through APs, and responses and acknowledgements may be returned through them. More generally, any node in an ad-hoc wireless network may be used as routing proxy to access or communicate with one or more other nodes in the network.
In one example, electric utility companies have been using mesh networks to automate the operation of the electric power distribution grid to provide a higher level of reliability and operational and maintenance efficiency. In many cases, substations can be largely automated, but distributed feeders may be much less automated.
The SCADA (Supervisory Control and Data Acquisition) system is an example of a system that can monitor and control distribution grid elements (e.g., switches, transformers, substations, feeders) via remote terminal units (RTUs) as part of a Distribution Automation (DA) network. Distribution Automation involves the remote monitoring of an electrical power distribution system and facilitates supervisory control of devices. DA also provides decision support tools to improve system performance.
SCADA back-office systems are typically designed to use static IP addresses to address DA equipment (capacitor bank controllers, switch reclosures, substation equipment, feeders, etc). These DA devices may be connected to the utility network via an Ethernet bridge (ebridge). The ebridges may be nodes in the wireless utility network with the egress point provided by one or more gateways or APs. The APs can connect to the back office server via a WAN.
The ebridges may find a route to an AP and obtain an IP address from the AP, for example, by using an IPv6 prefix with a MAC address combination or via Dynamic Host Configuration Protocol (DHCP) in IPv4 networks. When an ebridge obtains an IP address, it publishes a Domain Name System (DNS) record with its MAC address as the name. This allows a back-office system to resolve the IP address for a particular ebridge MAC address.
The use of dynamic IP addresses in a network allows a utility network to be divided into segments by subnet. For example, each AP may be assigned to a subnet. Thus, there may not be a need to publish any additional routing information. As devices join the network, these devices may be automatically reachable, as their dynamic IP addresses are in the same subnet. However, as mentioned, SCADA systems may only be configured to communicate with DA devices that have static IPv4 addresses.
Thus, a challenge is to find a dynamic route to a statically addressed DA device if the ebridge to which it is attached joins different networks due to changes in Layer 2 connectivity. Layer 2, i.e., the Data Link Layer, is one of the layers in the seven-layer OSI model. It responds to service requests from the Network Layer and issues service requests to the Physical Layer.
An ebridge may at any time choose a different AP than the one it is presently using for egress. When an ebridge joins an AP's network, the AP publishes routing information for the statically configured node attached to the ebridge. These routing advertisements preferably adhere to a standard protocol so that off-the shelf routers will work within the utility network system.
One example of providing routing advertisements is through the use of the Routing Information Protocol (RIP). RIP may be used because it is relatively simple to implement and may be supported by many routers. RIP is a distance-vector routing protocol that uses a “hop” count as a routing metric. A hop is the trip a data packet takes from one router or intermediate point to another in the network. The maximum number of hops allowed with RIP is 15. The metric (hop) field includes values from 0 to 14. That is, if the AP advertises a hop count of 15, the router will add one to that number and advertise a metric of 16, which designates an unreachable destination.
In a utility network, ebridges may switch APs if a second AP's Layer 2 routing cost, i.e. the sum of the costs of the links between the nodes used, becomes lower than its current primary AP, or it loses its route to its current AP. The nodes can typically register with an AP for a period of eight hours. Re-registration normally takes place at the expiration of the registration period. However, a node may switch to an alternate AP and register with it at any time by sending a registration message to the new AP. The registration message may cause the node to be configured with an IPv6 address. The registration message also notifies the AP of any devices with static IPv4 addresses that are connected to that ebridge.
When an ebridge switches APs, e.g., it switches from AP 1 to AP 2, the ebridge may not be able to send AP 1 a message that it is no longer using it. For example, the ebridge may have lost all routes to AP 1. A problem now arises in that both APs think the ebridge is registered with them, thus creating a registration ambiguity. In a network with nodes that employ dynamic IP addresses, the DNS server is typically able to resolve this problem. However, in this example, the SCADA system is not using DNS, and the IP address of the device connected to the ebridge is static, so a DNS lookup may not solve the problem. In addition, both APs continue to publish RIP updates for the device connected to the ebridge.